Photographic light-.sensitive materials make use of proteins and, in particular, gelatin as binders. For example, gelatin is used as the main component of silver halide emulsion layers, protective layers, filter layers, intermediate layers, antihalation layers, backing layers and subbing layers of photographic light-sensitive materials.
It is known that the mechanical properties of gelatin containing layers of photographic materials can be improved by addition of a hardening agent. In fact, the photographic materials are usually stored at elevated temperatures and humidity conditions or treated with various aqueous solutions having different pH's and temperatures, and gelatin layers which have not been treated with a hardening agent have poor water resistance, heat resistance and abrasion resistance.
Many compounds are known to be effective for increasing mechanical resistance of a gelatin layer by hardening. They include, for example metal salts such as chromium, aluminium and zirconium salts; aldehydes such as formaldehyde and glutaraldehyde; halogenocarboxylaldehydes such as mucochloric acid; aziridine compounds such as those described in U.S. Pat. No. 3,017,280; epoxy compounds such as those described in U.S. Pat. No. 3,091,537; halogenotriazine compounds such as hydroxydichlorotriazine and aminodichlorotriazine; compounds having vinylsulfonyl groups such as methylene-bis-vinylsulfone, divinylsulfone and methylenebis-vinylsulfonamide.
A group of hardening agents for photographic gelatin-containing layers which is particularly interesting has been disclosed in U.S. Pat. No. 4,063,952. These hardening agents are carbamoyl pyridinium compounds in which the pyridine ring carries a sulfo or sulfoalkyl group. These compounds have high water solubility, a fast hardening action for gelatin and low occurrence of post-hardening (post-hardening is a change in the degree of hardening caused by slow continued hardening of the gelatin). They belong to the group of "quick-acting" hardeners with which the light-sensitive photographic materials can be hardened to an optimum degree within a very short time.
U.S. Pat. No. 4,063,952 describes a method of preparation of said carbamoyl pyridinium compounds in which the pyridine ring carries a sulfo or sulfoalkyl group. According to that method, the alkali metal salt of a pyridine carrying a sulfo or a sulfoalkyl group is reacted with a carbamoyl halide in the presence of a mixture of dimethylformamide and methanol as solvents, and the reaction product (which may partly precipitate) is actively precipitated by addition of ethyl ether. Alkali metal halide, which is a by-product of this reaction, precipitates together with the carbamoyl pyridinium compound. Accordingly, U.S. Pat. No. 4,063,952 describes the hardening agent by the formula: ##STR1## in which R.sub.1 and R.sub.2 each represents an alkyl group, an aryl group, or an aralkyl group, or R.sub.1 and R.sub.2 together represent the groups required to complete a piperidine or a morpholine ring,
R.sub.3 represents hydrogen, methyl or ethyl, PA1 R.sub.4 represents methylene, ethylene, propylene or a single chemical bond, PA1 Me.sup.+ represents an alkali metal cation such as Li.sup.+, Na.sup.+ or K.sup.+ and PA1 X.sup.- represents an anion such as CI.sup.- or Br.sup.-.
The presence of an alkali metal halide in the hardening agent may cause problems in the photographic material using said hardening agent. In fact, the saline content in the photographic material is increased, thus increasing a tendency of the photographic material to stick when stored in rolls or sheets. Additionally, it is well known that halide ions, such as chloride, bromide or iodide ions may change the photographic characteristics of the silver halide emulsions.
J01-066,162 describes a synthesis of carbamoyl pyridinium compounds, in which the pyridine ring carries a sulfo or sulfoalkyl group. The process comprises forming the inner salt of a pyridine sulfonic acid in the presence of a tert.-amine compound and reacting the inner salt with a carbamoyl chloride. By this synthesis, hardening agents free of inorganic salts, e.g., NaCl and KCl, are obtained.
DD 290,879 describes a synthesis of carbamoyl pyridinium compounds in which the pyridine ring carries a sulfoalkyl group which comprises reacting a pyridine sulfonic acid with a carbamoyl chloride in a nitrile solvent in the presence of a tert.-amine compound. As nitrile solvents, acetonitrile, propionitrile, butyronitrile and benzonitrile are mentioned. The purpose of DD 290,879 is to provide a solvent in which the reactants and the tert.-ammonium halides obtained as by-products of the reaction are soluble, and the carbamoyl pyridinium compounds are insoluble. It has been found, however, that the carbamoyl piridinium compounds still contain, after the reaction, significant amounts of tert.-ammonium halides, which can be eliminated by subsequent washing steps with the nitrile solvent. The yield of pure carbamoyl pyridinium compound is consequently lowered.